routing a continuous dado around corners

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Can anyone suggest a good way to route a continuous 1/4" x 1/4" dado on all four sides of a 3 1/2" square (16" long) block of wood? The cut is complicated by the fact that 2/3's of the 16" length is tapered on all four sides so I can't just use a simple stop block as I would if the stock were not tapered. I made a simple sled for my router table which allows me to visually line-up each cut with the previous cut in--combination with the edge of the router bit's path through the plywood base--but it's time-consuming and not perfectly accurate. At least one dado ends up being a few thousands higher or lower than its neighboring dado. Since I'm using a 1/4" inlay all around this uneveness creates visual issues. I think I need to devise a fence/guide instead, as that would guarantee each cut is equal distance from the bottom of the stock, and then use a hand held router. But 3 1/2" isn't long enough for a fence so I would have to extend it somehow while also stabilizing the wood block somehow. Any ideas?

This is a dado gross grain at 90 degrees to the 16" length? As in banding that runs around a table leg?

If so, you are making square stock before you taper, so do the dado before the stock is tapered. Then do the taper, leaving the dado at the correct placement and depth. This requires the dado to be deeper than you doing so after the taper, depending upon where you need the dado.

As for the router method; after tapering, your inaccuracy is caused most likely by one (or more) of three things: the angle of the sled is slightly off, or the taper is not a consistent angle from face to face, or the stop block you register from is not contacting at the exact same point on the stock as you rotate it.

In looking at these sorts of problems, it is helpful to determine what things are easier to control than others. Know what you can reliably control and take control, leave the rest aside.

As our buddy Donald Rumsfield says, There are known knowns, and known unknowns, and also unknown unknowns.

Thanks to all for the feedback. David R. I hope you got my email. Rich, I like the concept but not sure I'm visualizing it correctly. Looks like you're suggesting a jig/pattern that fits over the square end (to be precise, a 3 1/2" cubed end) of my 16" piece. The stock and jig would be horizontally positioned. The jig would have a straight edge, i.e. pattern, 90 degrees to the stock for the bushing to ride against and, on top, a "shelf" extending out from the stock enough to accommodate the base of my router as it travels the 3 1/2" distance. For each new side the piece would be turned 90 degrees and the jig/pattern reinserted on the end. That would work but I would still need some type of micro adjustment to align the router bit perfectly with the previous cut. Even a minor deviation would throw off the alignment of adjoining dados on opposing sides. Am I barking up the right tree here or am I totally off base?

Like David said:
1. Square stock 3.5' x 3.5' x 16"
2. Using a miter gauge or sled on the table saw cut the dadoes around the bottom of the leg.
3. Glue the contrasting wood into the dadoes.
4. Taper the leg.

Reverse 4 and 3 if you need to, then plane the inlay to match the taper.

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